The present invention relates to a hydraulic device of the kind including a cylinder in which a piston slides in a leaktight fashion so as to delimit at least one chamber intended to be linked to a hydraulic circuit.
The invention especially concerns such a device intended to be used in association with motor-vehicle equipment. The invention may relate, for example, to a hydraulic transmitter for control of a clutch or for actuating the brakes of the vehicle.
In these applications, the device, with other components such as ducts, a reservoir, one or more hydraulic receivers, etc, forms part of a hydraulic circuit the correct operation of which requires that, after it has been filled with liquid, it is purged of any air or other gas which it might contain.
To that end, it is known to place the hydraulic chamber delimited by the piston under negative pressure with a view to making the liquid circulate, and to expel any air bubbles, especially by using the passage for automatic resupplying of the cylinder with liquid which is linked to the outside so as to communicate with the reservoir.
However, in order for this passage to be clear, it is necessary for the piston to be either in its rest position, or in a position close to it, to which it is normally drawn back by a return spring of rated value arranged inside the cylinder.
In normal use of the transmitter, the driver has to overcome the return force in order to shift the piston and cause the hydraulic circuit to be placed under pressure.
When the chamber is placed under negative pressure with a view to purging the circuit, the piston has a tendency to be displaced forward counter to the force which is applied to it by the return spring. If the application of negative pressure causes a sufficient forward displacement of the piston, the resupply passageway is blocked and the purging circuit is interrupted.
Furthermore, it is impossible to increase the rated value of the force exerted by the return spring, since too high a value impairs the ergonomics and comfort of the transmitter in normal use.
In order to remedy these drawbacks, the invention proposes a hydraulic device of the kind including a cylinder in which a piston is slidingly mounted in leaktight fashion in the body of the cylinder so as to delimit at least one hydraulic chamber intended to be linked to a hydraulic circuit, and is linked to the front extremity of an actuating rod which projects axially rearward outside the cylinder body and on which an actuating force can be applied in order to cause forward displacement of the piston with respect to the cylinder body, wherein it includes a temporary end stop which limits the forward displacement of the piston and which intervenes until the piston is first actuated by means of the actuating rod.
By virtue of such a configuration, the forward displacement of the piston resulting from the chamber being placed under negative pressure is limited to a predetermined range of travel which is chosen to correspond to a position of the piston in which the radially inner extremity of the resupply passage is always open.
According to other characteristics of the invention:
the temporary end stop limits the axial forward displacement of the rod, with respect to the cylinder body, in such a way as to be arranged outside the device in order to be easily put in place or removed;
the temporary end stop is a component affixed onto the actuating rod or onto the cylinder body, which is capable of occupying a fitted, end-stop position and a retracted position.
the end stop is preferably affixed by elastic clipping in order to allow it to be put in place and removed without any special tool;
the end stop includes means forming a ramp which interact with a facing portion of the cylinder body and/or of the actuating rod in order to cause automatic unclipping of the end stop under the effect of the actuating force applied to the actuating rod which entails a relative forward displacement of the rod with respect to the cylinder body;
the end stop includes a central fixing segment in the form of an elastic clip capable of being clipped elastically, along the radial direction, onto a complementary segment of the actuating rod;
the end stop, at at least one of the extremities of its central fixing segment, includes a free end segment shaped into an inclined ramp and which is capable of interacting with a facing surface of the rear extremity of the cylinder body or of a radial shoulder of the actuating rod so as to cause the end stop to be disengaged in an overall radially outward direction;
in position, fitted on the actuating rod, the end stop is mounted for sliding axially along the actuating rod.
Other characteristics and advantages of the invention will emerge on reading the detailed description which follows and which is given by way of non-limiting example.